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Jake Paul stripped all the way down to his birthday suit for his 25th cake day on Monday ... going buck naked for a sexy photo with his model girlfriend.

The Problem Child just posted the spicy pic to his social media page ... showcasin' his naked bod while a bikini-clad Julia Rose did just enough to cover up his junk.

Jake rocked a hat and some expensive jewelry -- and not a thing more -- while Rose had her hands draped all over him.

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